70's fender deluxe reverb hiss in vibrato channel

[pontiacpete]

I'm completely stumped here. The owner of the amp likes to play this amp wide open, volume, treble and bass cranked, but when doing this, the vibrato channel hisses. There is a significant difference in the amount of hiss between the 2 channels. I've tried practically everything from plate resistors, input resistors, vol and treble pots, all caps in that channel, moved wires around and changed tubes over and over again(even tried lower gain pre amp tubes).
What else is there?

Thanks in advance for any suggestions.

pete

[soundguruman]

There IS hiss in the design, metal film resistors in the audio path can help reduce it.
using 1 watt plate resistors will help reduce thermal noise. Bulk metal foil resistors are the lowest noise...
A plate to grid capacitor, 7 pf 1000V at V1A can help a bit too. A grid to ground cap at V1A. PV uses 39 pf, can reduce some of it...
You can wrap the grid wire V1A with the plate wire, this can produce some feedback to help reduce it.
Basically your HISS noise comes from V1a, and is amplified by the rest of the stages. So anything to reduce the noise of that stage will be carried through.
A lower noise tube, will also be lower gain...and so there is a point where the gain will be too little.

[R.G.]

A word about resistor types. Any time someone posts "hiss" the issue of resistor type comes up. Metal film and wirewound have zero excess noise; translated into Non-Techie, that means that they have the lowest noise possible given their resistance and the temperature they work in. Carbon comps have excess noise that depends on the resistor, the age, the materials, the history of the actual part, the phase of the moon, yada, yada.

Once those two things are said, the discussion then centers on carbon comps having better tone, and possibly that metal film/wirewound sound "sterile".

And that is true - ish. Carbon comps have a slight second order distortion when the signal alternation across them is more than maybe 50V-100V peak to peak, due to their voltage coefficient of resistance. The resistance of a carbon comp actually changes depending on how much voltage is across it. So if the voltage across it changes a lot, its resistance changes and the signal is distorted. Guitarists seem to like the sound.

But this only happens for big signals. The input stage of an amp does not have signals big enough to make much voltage-coefficient distortion. Input stages also produce almost all the audible noise, for the reasons SGM just stated.

So my favorite trick is to re-fit the input stages in all metal film for lowest noise. This gets the resistor noise down to where the noise you hear is generated in the tube, the socket contacts, the jack contacts, solder joints, etc. This has little or no effect on the carbon comp mojo sound, which is generated elsewhere.

[JoeM]

I'm completely stumped here. The owner of the amp likes to play this amp wide open, volume, treble and bass cranked, but when doing this, the vibrato channel hisses. There is a significant difference in the amount of hiss between the 2 channels. I've tried practically everything from plate resistors, input resistors, vol and treble pots, all caps in that channel, moved wires around and changed tubes over and over again(even tried lower gain pre amp tubes).
What else is there?

Thanks in advance for any suggestions.

pete

Are you sure there is a problem in the amp? At least it seems to me, even with low noise resistors which will no doubt help, any Fender cranked all the way, and with the treble at max is going to hiss. I would expect you could reduce it, but at extreme levels some hiss would be normal.

[soundguruman]

The very start of the audio path, the grid resistors, the first plate resistor, this is where majority of noise comes from. It's all amplified from there on.
Metal film resistors, bulk foil, will become the new standard for all guitar amplifiers.
Sooner or later the carbon resistors will be recognized as the source of noise, and people will stop using them.

The "mojo" is the fact that these carbon comps cost 1/2 cent EACH. No wonder everybody wants to sell them. But they are inferior for audio circuits.

Notice what kind are being used in high dollar high gain amps? It's metal film. That's cause the gain is high and the noise must be minimum.
So you can install all the "mojo" you want, and suffer with the noises.

INPUT GRID RESISTORS- I think away from the circuit board entirely, away from the rest of the components, mounted on the input jack, certainly NOT mounted on the circuit board. I think as far as noise goes, Leo had the right idea.

[loudthud]

Fender style eyelet boards can produce noise due to moisture absorbson. The cure is to dry them with a hair dryer or set them outside in the sun for a couple of hours.

[pontiacpete]

I have read a fair amount of posts jhere about the resistors and hiss. I unfortunately don't have any metal film resistors at the moment, but I've have tried replacing all input and plate resistors with carbon film resistors which in the past has cured such problems. Now I have some 1 watt metal oxides on the plates of the trouble channel and no change. That leads me to the obvious question-- why is it happening in the one channel and not the other?

I am going to go with loudthuds suggestion to dry out the board while I wait from my metal films to come in.
Thanks guys

[JoeM]

....

That leads me to the obvious question-- why is it happening in the one channel and not the other?...

From your first post I had the impression both channels hiss, just more on the Vibrato channel. There's more gain on the Vibrato channel, and possibly the 10pf in parallel with the 3.3M at the reverb mix stage might contribute too (High frequencies there). But I'm still wondering, at more normal volume settings, does the amp have objectionable hiss? I checked my DRRI, and it's noisy at extremes (dimed), but certainly not a problem at any level I normally use the amp.